Ultrasonic study of the nematic-isotropic phase transition in PAA

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Abstract
We present an ultrasonic investigation of the nematic-isotropic phase transition in p-azoxyanisole (PAA). Our measurements were performed at several frequencies ranging from 0.8 to 5 MHz as a function of temperature and, in the nematic phase, as a function of the orientation of the liquid crystal with respect to the ultrasonic wave vector. In both phases, our results are within the ωτ ⪡ 1 regime where τ is the acoustical relaxation time. In the isotropic phase, the results may be quantitatively interpreted using the dynamic heat capacity theory which predicts for α, the ultrasonic absorption, a critical exponent of 1.5. In the nematic phase, we find a critical exponent of ∼ 1 for Δα(T), the attenuation anisotropy, and α(T). This result seems to show that the Landau-Khalatnikov mechanism is the dominant contribution in the temperature range investigated which corresponds to Tc— T values from 1 °C to 20 °C. For comparison purposes we also include some data for PCB that we published some time ago